Demands and requirements of users for wireless communication services have continually increased with the increased spread of mobile communication, and particularly, demands on various high speed multimedia services have been substantially increased. Accordingly, the importance of quality of service (QoS) of the communication service has been emphasized, and it is an essential function of the wireless communication system to support the QoS in the wireless communication service.
In comparison to a wired communication system, the wireless communication system imposes additional challenges due to the fact that the wireless channel is unreliable and its characteristics vary in time and by location. Cases of generating hostile wireless communication environments include the case in which a user moves fast during communication, the case in which a communication channel has a plurality of multi-paths other than the line of sight (LOS), and the case in which strong interference is provided by a neighboring system. When the channel environment is hostile as described, the receiver has a problem in the channel estimation, and phase error occurs during the channel estimation process to deteriorate the receiving performance of the system.
The phase error present in the received signal in the wireless communication can be generated by various reasons such as a fading channel caused by the Doppler effect or a multi-path, or performance of a receiver that is not ideal.
Existing and future communication systems must allow high data rates and provide stable services under hostile channel environments. It is needed for the physical layer of the communication system to provide excellent link performance in order to allow stable high data rates in the wireless communication system. In order for the physical layer to provide excellent link performance, a transmitter must transmit the signal by using a modulation method having excellent performance, and a receiver must eliminate phase error and noise that are generated when the signal is passed through the radio communication channel by using an excellent synchronization algorithm and a channel estimation algorithm.
The orthogonal frequency division multiplexing (OFDM) scheme is a transmission method for providing excellent performance under a hostile channel environment, and is widely used in the wireless LAN and the WiMAX/WiBro systems since its performance is excellent in the condition in which the channel characteristic is determined by multi-paths (e.g., office or urban area) and its frequency usage efficiency is great. In order to increase the transmission efficiency in the OFDM system, a high-order modulation method such as the 64 QAM is used when the channel environment is good. However, when the channel environment is hostile, the symbol error rate is increased and the high-order modulation method such as the 64 QAM cannot be used. Here, even when the signal to noise ratio (SNR) is large, if a significant amount of phase error is present in the received signal, the symbol error rate is great so that a low-order modulation method such as the 16 QAM or the quadrature phase shift keying (QPSK) must be used.
To overcome the influence of the hostile channel environment, the receiver can use a synchronization algorithm or a channel estimation algorithm with excellent performance. However, it is limited for the receiver to remove the influence of the channel environment since the signal structure is determined by the transmitter. For example, when the channel environment is substantially changed as time is passed or when the length of a preamble or a pilot signal used for synchronization or channel estimation is not sufficiently long, the estimation of the channel fails to be valid within a short time or a synchronization or channel estimation error occurs, and the limitation of the receiver shows up as noise and phase error in the received signal. Since the receiver has a limit in overcoming the influence of the channel environment, it is required to transmit the signal so as to minimize the signal distortion by the phase error that is generated while the transmitter passes through the channel, in order to allow stable data transmission with a high data rate in the hostile channel environment.
Many communication systems use the high-order modulation method such as the 64 QAM so as to transmit data at a high data rate. FIG. 1 is a conventional 64 QAM constellation on the I-Q (in phase-quadrature) plane. Since 6 bits are required to express numbers of 64 cases, it is possible for the 64 QAM to transmit 6 bits by transmitting a symbol.
However, when the SNR of the received signal is low or the received signal has phase error, the transmitted symbol can be mistakenly determined to be an adjacent symbol and hence it is impossible to use the high-order modulation method such as the 64 QAM.
FIG. 2 is a received signal caused by the 64 QAM displayed on the I-Q plane when phase error and white noise are present. Referring to FIG. 2, the scattering of the received symbols without directivity from the original symbol location is the influence of the white noise, and the influence of the white noise to the received symbols is dominant in the symbols located close to the origin. On the other hand, in FIG. 2, the symbols that are distant from the origin for which the influence of the phase error is dominant are scattered to form an arc with reference to the origin, and the scattering degree is in proportion to the distance from the origin. As described above, the phase error applies more influence to the symbol that is far from the origin than the symbol that is near the origin differing from the white noise that applies the same influence to all the symbols. Therefore, when there is phase error, it is difficult to apply the high-order modulation method such as the 64 QAM for high-speed data transmission.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.